This thread came up a lot when I looked around about this, I intended to post this several months ago. Unfortunately I wanted to upload the mod to mod sites and do some editing, but I haven't gotten around to do it yet.

Anyway, I have created a little mod which fixes these birthsigns, here is a short description:

Vanilla Birthsigns for OpenMW by Klorax changes the behavior of the attribute enhancing birthsigns (the Lady, the Lover, and the Steed) to match that of original Morrowind.

This is accomplished by replacing the constant magic effect with base attributes upon finishing character generation (i.e., when leaving the census office building). When this happens the player is notified. Thus you need to start a new game to use this addon.

In original Morrowind these constant magic effects for attributes are treated as base attributes. This is inconsistent to any other constant magic effect (i.e., spell or enchant) for attributes. OpenMW removes this inconsistency, which is good IMHO, but this makes the original birthsigns behave differently (mostly less powerful), and this addon aims to fix this.

This means that the provided endurance will now count for level up health. All relevant attributes (agility, endurance, personality, and speed) will now count towards skill level caps for governed skills. The attribute base values (through leveling) will not be able to get past 100.

Thx for this. It is sad that i have to install a mod for correct behaviour.

Glad to help, but sorry you feel that way! I really agree with OpenMW's change. The original behavior is strange since it makes a game mechanic work very different in one specific case. However, I understand that you and many others wanted the birthsigns themselves to work like before, that is why I made this mod!

Changing the base attribute is not unique to birthsigns. Picking your favourite attribute during character creation also adds +5 to your base attribute, the same way as vanilla implements birthsigns.

I find the logic of when OpenMW applies +25 and when it doesn't to be arbitrary and hard to figure out. And, as pointed out elsewhere, the current broken OpenMW birthsign implementation causes issues for faction requirements and training. And by "broken", I mean different from vanilla Morrowind.

IMNSHO, OpenMW should implement birthsigns the way Morrowind does, and then we need a mod to fix the game balance issue. My preference would be a +10 nerf, the way Oblivion does it. Of course, by using a mod, everyone could get a fix that suits them.

IIRC, birthsigns are implemented in the game data by being a constant magic effect, but in the vanilla engine, for every constant magic effect it does a check to see if it was provided by a birthsign, and if it was, it pretends it isn't a constant magic effect after all.

Changing the base attribute is not unique to birthsigns. Picking your favourite attribute during character creation also adds +5 to your base attribute, the same way as vanilla implements birthsigns.

I find the logic of when OpenMW applies +25 and when it doesn't to be arbitrary and hard to figure out. And, as pointed out elsewhere, the current broken OpenMW birthsign implementation causes issues for faction requirements and training. And by "broken", I mean different from vanilla Morrowind.

In original Morrowind and OpenMW, the relevant birthsigns adds spells that are constant effects (picking class attributes does not do this). In Morrowind, constant effect spells enhances the base attribute, but does not count towards level up, and so on. In Morrowind, these are never treated the same as base attributes, except for this one time. OpenMW always treats them differently, i.e., never makes this exception.

In vanilla Morrowind, and for that matter vanilla Oblivion, stat increases that are of type "Ability" modify the base stat, whereas switching it to type "Curse", "Power", "Enchantment", "Disease" or "Spell" will cause it to function as a buff or debuff in the form we're used to. Furthermore, is is for Attribute, and perhaps Skill, modifications ONLY. Birthsigns are Abilities, which means they modify the base rather than applying a Fortify effect like the game claims. They are not, however, the only Abilities to do this. For example, look at the mod "Lazy's Guild Rewards -- Abilities." LazyPilots had to script the abilities to be temporarily removed when you sleep, because otherwise they would apply to attribute caps on level up. This is solely because they are Abilities. OpenMW does not and quite frankly should not replicate this behavior, which I suspect only exists because of the bug Bethesda never fixed that makes Restore Attribute effects only restore up to the base, which would render unremovable effects like abilities that fortify attributes worthless.

In short, the vanilla behavior, which is so accepted now, is itself a hacky workaround, and that's a problem for attempting to replicate it.

I am, however, quite glad there is now a mod to get the best of both worlds.

That is not a valid reason to not replicate the vanilla behavior. It affects more than just restore attribute, it affects how you level and your training, as if it were an inherent bonus, just like how different races have different base stats, and that's the way it should be. (In other words: what happens in chargen acts like it's fundamental.) It doesn't matter whether you consider it bad behavior or a workaround or not, vanilla parity is OpenMW's first objective.

Compared to the many other things that have "fixes", why can't there just be a toggle in the config/launcher to allow user to use vanilla behavior if they so wish? I'd probably still use the "corrected" behavior, but I do agree that it doesn't make sense to move away from vanilla behavior on this one thing, when I'm sure there were other times when something was kept to vanilla behavior when it could have been improved. It makes it seem like changes are arbitrarily decided as to what should and shouldn't be kept the same as vanilla. If something is going to move away from vanilla, there should at least me a "master toggle" to make all "QoL" changes default to vanilla behavior.